In the manufacture of microstructures, for example microelectro-mechanical structures (MEMS), etching processes are used to remove sacrificial (i.e. unwanted) areas of material. MEMS have found applications in inertial measurement, pressure sensing, thermal measurement, micro-fluidics, optics, and radio-frequency communications, and the range of possibilities for these structures continues to grow. Sacrificial layers are initially employed in the construction of the MEMS and then subsequently removed with an etch step, which allows the released structure to operate as designed. In order to produce reliable structures the release etch step is required to remove the sacrificial layer without etching the surrounding material. Ideally the etch of the sacrificial layer should have no impact at all on the remaining structure.
Many materials are known to be employed during the manufacture of MEMS e.g. silicon, silicon dioxide (SiO2), silicon nitride (Si3N4), aluminium and photoresist to name but a few. Some of these materials are employed as the sacrificial materials while others are employed to define and hence form the MEMS. It is not uncommon during the manufacture of a MEMS for more than one sacrificial etch step to be employed. For example a film may initially be employed as a mask during a first sacrificial etch process and then be subsequently etched as the next sacrificial layer. In any release etch it is therefore highly desirable for there to be a high etch selectivity between the sacrificial layer and the surrounding materials.
A commonly employed approach to quantifying the selectivity of materials is to etch blanket films of those materials using the same etch arrangement and then compare the amount of material that has been removed. This technique is widely used and gives very useful information. However, in practice etch selectivity can be found to be dependent upon the materials present and the manner in which they have been deposited, the etch characteristics itself, and any subsequent treatments carried out on the MEMS.
By way of example, hydrogen fluoride (HF) vapour etching is commonly used to remove sacrificial areas of silicon dioxide in the manufacture of MEMS. This etch is a chemical etch, with no plasma being required, is performed at a process chamber pressure in the range of 18 T to 150 T and normally requires heating so as to achieve an operating temperature between 25° C. and 70° C. A catalyst is required for the HF vapour etch of silicon dioxide (SiO2) to proceed. Water (H2O) is often employed as the catalyst since this gives a fast and controlled etch, although alternatively catalysts known in the art include alcohols, methanol, ethanol and propanol. However, water (H2O) (along with silicon tetrafluoride (SiF4)) is a by-product of the reaction process and this means that the inherent etch characteristics can have a significant influence on the etch taking place. Therefore careful control of the process conditions is required.
The hydrogen fluoride (HF) vapour etching of silicon dioxide is known to exhibit high selectivity to many common films. For example, the theoretical selectivity to silicon and aluminium is high and no etching or corrosion is expected. However, the above described process conditions are also compatible for the hydrogen fluoride (HF) vapour to etch silicon nitride (Si3N4). Therefore in practice it can prove difficult to achieve a high selectivity between silicon dioxide (SiO2) and silicon nitride (Si3N4) layers during a hydrogen fluoride (HF) vapour etch.
It is therefore an object of an embodiment of the present invention to provide apparatus and methods for the hydrogen fluoride (HF) vapour etch of silicon dioxide that exhibits increased selectivity to silicon nitride as compared to those techniques known in the art